1. Field of the Invention
The present invention relates to a gypsum-based panel such as plasterboard, for example, and a method for making such a gypsum-based panel.
2. Related Art
Plasterboard is well known for its uses in the construction industry. It may be used in interior wall and ceiling construction. Its main advantages over more traditional materials are its low manufacturing cost, low installation cost and fire retardance.
Plasterboard typically has a gypsum-based core sandwiched between two sheets of liner paper. However, it is possible to form a similar product using different liners, or with no liners. For this reason, the more general term “gypsum-based panel” is used herein. It is to be understood that plasterboard is one class of gypsum-based panel.
Plasterboard is typically formed in a continuous process in which a slurry is provided by mixing dry ingredients with water and a foaming agent. Mixing techniques for providing the slurry vary by region, but generally the slurry is mixed and then continuously deposited between two continuously supplied sheets of liner paper. One of the sheets of liner paper (typically the face paper) is folded and bonded to the other sheet of liner paper (typically the backing paper). The product of this process is pressed to the required thickness by a forming plate. The slurry is then allowed to set between the two liner paper sheets. The continuously-produced board is then cut into panels of a predetermined length which are then dried in kilns to remove excess water.
Typically, the dry ingredients of the slurry include calcium sulphate hemihydrate (CaSO4.0.5H2O), also known as calcined gypsum, acid-modified starch for promoting the bond between the liner paper and the core, and optional other additives. These are mixed with water and foam or a foaming agent. The purpose of the foam is to include voids in the slurry in order to reduce the density of the set core of the plasterboard.
The calcined gypsum undergoes a hydration reaction in the presence of water to form calcium sulphate dihydrate (CaSO4.2H2O), or gypsum. Needle-like crystals of gypsum form and it is the resultant network of these crystals that lends the finished product the basis for the compressive strength of the core.
The acid modified (thinned) starch added to the slurry promotes the adhesion of the liner paper to the core. It is believed that the acid modified starch migrates with water during the board manufacturing process to the paper-core interface to promote adhesion, as explained in U.S. Pat. No. 2,207,339, in which an acid modified starch is prepared by modifying raw starch using dilute nitric acid.
It is also known to apply starches directly to the liner paper, to improve adhesion of the liner paper to the core. See, for example, U.S. Pat. No. 4,051,291, U.S. Pat. No. 4,117,183, U.S. Pat. No. 4,119,752.
WO 02/12141 discloses the use of starch in plasterboard and explains that the starch migrates to the core-paper interface to help the liner paper bind to the core.
U.S. Pat. No. 3,989,534 suggests that a film former may be added to a gypsum slurry in order to improve the stability of a foam introduced into the slurry to reduce the final density of the product. Suitable film formers are cold-water soluble organic compounds such as pregelatinized starch, guar gum or xanthan gum.
FR-A-1412596 discloses the use of dextrins (a form of starch) in jointing compounds for use with prefabricated gypsum boards. Similarly AU 9221039 discloses the use of a combination of PVA, methyl cellulose and yellow dextrin (see below) in a jointing composition.
U.S. Pat. No. 4,169,747 discloses an accelerator composition for plasterboard, the accelerator comprising finely ground hydrated gypsum and an additive for preserving the gypsum from calcination. The additive may be lignosulfate, starch, dextrin or sucrose.
U.S. Pat. No. 6,251,979, U.S. Pat. No. 6,340,388, U.S. Pat. No. 6,391,958 and U.S. Pat. No. 6,403,688 briefly discuss the use of starches or dextrins as an adhesive agent to promote the bonding of liner paper to the core in plasterboard.
DE-A-3246534 also disclose the use of adhesives in wallboard manufacture, the adhesive acting to adhere the core to the liner paper, and being one of starch, dextrin or synthetic resin. JP-B-72-051086 contains similar disclosure.
WO 97/35888 discloses methods for starch degradation, for use in wallboard.
U.S. Pat. No. 6,319,312 discloses a plasterboard formulation including an expanded mineral such as perlite in order to reduce the amount of gypsum required. The formulation requires a combination of synthetic binders that form a crosslinked network in the plasterboard. The combination of synthetic binders is a vinyl acetate emulsion in the form of poly(vinyl acetate) particles in polyvinyl alcohol and water, a source of boron (e.g. borax) and starch. It is suggested that the starch could be dextrin, although this is not exemplified. The use of perlite (in effect, a lightweight aggregate) in the board required the use of the synthetic binders in order to bond the core. Failure to use the synthetic binders would likely have led to a core with unsuitably low strength. It has been found that the formulation exemplified in U.S. Pat. No. 6,319,312 is not effective in providing a suitable plasterboard structure, for reasons that are not well understood at this time. Furthermore, it is not preferred to use synthetic binders such as PVA in plasterboard made using normal foamed gypsum slurry, since the synthetic binder is not cost-effective for large-scale industrial plasterboard manufacture.
WO 99/08978 discloses the addition of trimetaphosphate ions to a plasterboard slurry, to provide an increase in strength of the core. In addition, that document discloses that the typical use of a nonpregelatinized starch (e.g. an acid modified starch) to promote the paper-core bond may not be sufficient to prevent weakening of that bond when the board becomes wet. Thus, as well as the use of trimetaphosphate ion, WO 99/08978 discloses the addition of pregelatinized starch to the slurry. The result of this is that the weakening of the paper-core bond is reduced. The mechanism for this beneficial effect is not explained in WO 99/08978, but it is observed that the pregelatinized starch becomes distributed throughout the core. Raw starch is pregelatinized by cooking in water at temperatures of at least 185° F. (about 85° C.). The pregelatinized starch is included in the slurry in an amount between 0.08-0.5% by weight, based on the weight of gypsum.
U.S. Pat. No. 5,922,447 discloses the uses of a partially cooked starch in wallboard. The wallboard core contained both gypsum and perlite. The starch binds the gypsum particles and perlite spheres and does not tend to migrate to the paper-core interface. The starch is partially cooked in water at temperatures between 150° F. and 185° F. (between about 65° C. and about 85° C.). The starch used is pearl starch (in this case a combination of starch made from corn, potato and/or wheat stock), although acid-modified starches are also suggested.
One measure of the core strength of plasterboard is nail pull resistance. Methods for testing the nail pull resistance of plasterboard are set out in ASTM C473-03 (Standard Test Methods for Physical Testing of Gypsum Panel Products). Ideally, for many applications, plasterboard should have a low density (usually referred to by the weight per area for a particular board thickness, e.g. pounds per thousand square feet (lb/MSF)) and a high nail pull resistance.
U.S. Pat. No. 6,783,587 discloses a plasterboard in which starch is added in an amount between 1.5-3.0% by weight, based on the amount of gypsum, in order to increase the nail pull resistance of the board. The starch used is acid modified starch, of the type typically used to promote binding of the liner paper to the core.
US-A-2003/0084980 discloses a plasterboard formulation including acid-modified starch and a starch crosslinking agent (e.g. type N hydrated lime). The cross-linking agent is believed to lock some of the starch in the core, the remainder migrating to the paper-core interface to promote bonding.
US-A-2003/0092784 discloses a polymer-reinforced gypsum material, the polymer being crosslinked in situ. The polymer used is a synthetic polymer such as PVP (poly(vinyl pyrrolidone)) and the crosslinking component is PSS (poly(sodium 4-styrene sulfonate)).
U.S. Pat. No. 5,879,825 discloses the addition of an acrylic polymer composition, having a glass transition temperature of 15° C. or greater, to plasterboard in order to increase the core strength of the plasterboard.
US-A-2004/0092625, US-A-2004/0092624 and U.S. Pat. No. 6,841,232 disclose a gypsum based composite structure such as plasterboard with a cellulose ether additive (other than carboxymethylecellulose (CMC)), in order to improve nail pull resistance and flexural strength.
US-A-2005/0126437 discloses a wallboard core composition including a substituted starch. The properties of the starch (degree of substitution, degree of polymerization and viscosity) are selected so that the starch does not dissolve in water at the slurry mixing temperature, but does dissolve at higher temperatures. In this way, excessive migration of the starch to the core-paper interface is avoided.